This invention relates generally to the control of pollutants emitted from a combustion process. More particularly, this invention relates to an apparatus and method for decreasing the concentration of contaminants present in a flue gas stream emitted by a fossil-fuel fired boiler by using a hybrid electrostatic precipitator.
The 1990 amendments to the United States Clean Air Act require major producers of air emissions, such as electrical power plants, to limit the discharge of airborne contaminants emitted from combustion processes. In most steam power plants in operation today, fossil fuels (such as petroleum or coal) are burned in a boiler to heat water into steam. The steam drives electrical turbines, which generate electricity. These fossil-fuel fired boilers, however, emit highly polluting flue gas streams into the atmosphere. These flue gas streams typically contain noxious gaseous chemical compounds, such as carbon dioxide, chlorine, fluorine, NOx and SOx, as well as particulates, such as fly ash that is a largely incombustible residue that remains after incineration of the fossil-fuel.
To date, many devices have been used to reduce the concentration of contaminants emitted by fossil-fuel fired boilers. One of the most effective devices is the electrostatic precipitator or ESP. An ESP is a device with evenly spaced static conductors, typically plates, which are electrostatically charged. When flue gases are passed between the conductors, particulates in the flue gas become charged and are attracted to the conductors. Typically, twenty to sixty conductors are arranged parallel to one another, and the flue gas stream is passed through gas passages formed between the conductors. The particulate layer formed on the conductors limits the strength of the electrostatic field and reduces the performance of the ESP. To maintain performance, the conductors are periodically cleaned to remove the collected particulates.
There are two types of ESPs, dry and wet ESPs. A dry ESP removes particulates from the conductors, by shaking or rapping the conductors and collecting the removed particulates in a dry hopper. A wet ESP removes the particulates by washing the particulates off the conductors and collecting the removed particulates in a wet hopper.
Dry ESPs, however, have a number of shortfalls. First, when the conductors are rapped, some of the particulates are re-entrained in the flue gas stream. If the flue gas is vented to atmosphere after such a dry ESP field, any re-entrained particulates will vent into the atmosphere. Therefore, although dry ESPs are highly efficient, a certain amount of contaminants cannot be removed by the dry ESP. It has been shown through experimentation, that each field of a dry ESP can remove approximately 70% of the particulates entrained in a flue gas stream. Therefore, a number of dry ESP fields are typically arranged in series until a desired concentration of particulates is attained. An example of a dry ESP can be found in U.S. Pat. No. 5,547,496, which is incorporated herein by reference.
To date, wet ESPs have not been used in electric power stations. However, existing systems for removing particulates using a series of wet ESP fields are well known in the industrial sector. An example of a wet ESP is disclosed in U.S. Pat. Nos. 3,958,960 and 3,958,960, which are incorporated herein by reference. A problem with these systems is that the introduction of too much moisture into the flue gas leads to moisture saturation of the flue gas. This tends not to be a problem in industrial plants, as there is little or no gaseous chemical compounds present in the flue gas stream that can dissolve in the moisture to form acidic solutions. However, in combusted fossil-fuel flue gas, the saturated flue gas condenses and combines with the gaseous chemical compounds present in the flue gas to form highly corrosive acid solutions. To limit corrosion of the system by these acids, the system must be lined with acid inhibitors and include induced draft fans.
A system for removing particulates using a series of dry ESP fields and a wet ESP field is disclosed in U.S. Pat. No. 3,444,668, which is also incorporated herein by reference. This system removes particulates from a cement manufacturing process. This process does not address problems specific to fossil-fuel fired boiler emissions, such as removing contaminant gaseous chemical compounds present in a combusted fossil-fuel flue gas stream.
Furthermore, systems that position a wet ESP field upstream of a dry ESP field, such as that disclosed in U.S. Pat. No. 2,874,802, which is also incorporated herein by reference, do not sufficiently remove contaminants from a gas stream or address the above described problems.
In view of the foregoing, it would be highly desirable to provide an efficient system for decreasing the concentration of contaminants within a flue gas stream emitted by a fossil-fuel fired boiler, while addressing the above described shortfalls of prior art systems.
According to the invention there is provided an apparatus for decreasing the concentration of contaminants within a flue gas stream emitted by a fossil-fuel fired boiler. The apparatus includes a wet electrostatic precipitator (ESP) field disposed along a combusted fossil-fuel flue gas stream path downstream of a dry ESP field. The wet ESP field includes a chamber having a flue gas inlet and a flue gas outlet, and at least one collection plate positioned within the chamber. The chamber also includes one or more wash nozzle positioned adjacent to the collection plate and a wet hopper positioned substantially under the collection plate. The apparatus preferably further comprises one or more cooling nozzles positioned near the flue gas inlet. The cooling and wash nozzles are fluidly coupled to a water source, while the wet hopper is fluidly coupled to either a pH adjustment module or a treatment processor.
Further according to the invention there is provided a method of decreasing the concentration of contaminants within a flue gas stream emitted by a fossil-fuel fired boiler. Contaminants are electrostatically collected from a combusted fossil-fuel flue gas stream on dry and wet electrostatic precipitator (ESP) conductors, where the wet ESP conductor is disposed downstream of the first ESP conductor. The dry ESP conductor is then shaken to remove contaminants collected thereon, while the wet ESP conductor is washed to remove contaminants collected thereon. The wet ESP conductor is washed either continuously or intermittently, however, a continuous wash is preferred for ease of control.
To improve performance, water is preferably sprayed into the wet ESP inlet flue gas stream to lower the flue gas temperature. In one embodiment, the water sprayed into the flue gas stream to lower the temperature and used to wash the collection plates is acquired from an untreated water source. In another embodiment, the sprayed water is recirculated in a closed loop.